The present invention relates to a section or structured element of composite material, of the type knitted in three dimensions from natural or synthetic threads or fibers, impregnated with a resin.
More particularly, the invention concerns a process of producing a structured element that is knitted in three dimension and which has, in cross-section, a geometric form or cross-section composed of several rectangular surfaces connected together, at least two of these adjacent rectangular surfaces delimiting a reentrant angle of the geometric form.
Many section elements of this type are already known and the methods of knitting them in three dimensions are now currently employed.
However, many section elements produced up to the present time, such as those, for example, disclosed in the French Pat. No. 73/14956 and U.S. Pat. Nos. 4,183,232 and 4,346,741 of the applicant, are section elements having a cross-section in the shape of a solid of revolution whose length is limited by the very nature of their method of fabrication. U.S. Pat. No. 4,183,232 discloses a method which uses a plate on which are fixed rods or bars which define the periphery of the solid of periphery of the solid of revolution, this plate being driven in rotation so as to cause the bars to pass in front of knitting heads. The needles of these knitting head face threads in two transverse directions, the threads disposed in the third dimension being thereafter placed in the direction of the rods, by casting them individually in accordance with the method described in U.S. Pat. No. 4,393,669 of the applicant.
Apart from the aforementioned patents, there is also known a European Pat. No. 0,056,351 which describes a method comprising producing an element that is knitted in three dimensions having a certain thickness and whose marginal parts are divided into two in the direction of the thickness.
In order to produce a section element, it is then necessary to fold the marginal parts outwardly to obtain the desired shape, before impregnation. This method limits the shapes which may be obtained for the section of the section elements.
Various processes and machines are already known which permit realizing structured elements that are three diomensions and which have, in cross-section, a rectangular general form.
U.S. Pat. No. 4,526,026 (Krauland) describes an apparatus comprising two groups of thread guiding organs disposed at right angles to each other which permit the introduction of threads in a plane following two directions, X and Y, around threads extending at right angles following a third direction Z, and two rows of needles forming between them a right angle and disposed parallel and perpendicularly to the XY plane.
This apparatus permits making elements of rectangular cross-section, but if one wishes to modify the cross-sectional form, its configuration must be modified beforehand.
U.S. Pat. Nos. 3,904,464 and 4,038,440 (King) both describe an apparatus comprising two rows of parallel needles extending perpendicularly in an XY plane which is itself perpendicular to an assembly of parallel rods around which the rows of needles interlace threads.
With this apparatus, the cross-sectional form of the element obtained is rectangular and cannot be modified.